The genes associated with breast cancer, what it means to have a faulty gene, and who gets tested.

A fault (mutation) in certain genes can increase your risk of breast cancer. Women at risk of having a faulty gene are offered tests and screening.

Genes called BRCA1, BRCA2, TP53, PALB2 or PTEN are the main ones linked to a high risk of breast cancer. Everyone has these genes. If there's a fault in one of them, the risk of breast cancer is higher.

Faulty BRCA1 and BRCA2 genes also increase the risk of ovarian cancer. A blood test can identify whether you have a fault in one of these high-risk genes.

What is the normal risk of breast cancer?

In the UK, a woman's lifetime risk of developing breast cancer is 10-12%. Out of every 100 women, around 10-11 will develop breast cancer by the time they are 80 years old. It can affect anyone. You can get it even if you don't have a faulty gene.

If more than one person in a family has breast cancer, this doesn't mean the faulty genes are present. It could be chance, as breast cancer is common.

What's the risk if you have a faulty gene?

If you have a faulty gene, it doesn't mean you'll definitely develop breast cancer, but you are at higher risk.

Having a fault in one of the high-risk breast cancer genes raises the risk of developing breast cancer to between 40% and 85%.

Out of every 100 women with a faulty gene, between 40 and 85 of them will develop breast cancer in their lifetime.

Who is tested for faulty genes?

If you are diagnosed with breast cancer and have a strong family history of breast and/or ovarian cancer, you will usually be offered a blood test to see whether you have a faulty gene.

If the test is positive, this means you have a higher risk of developing cancer in your other breast and, with BRCA1 and BRCA2, developing ovarian cancer.

"Women who have a mother or sister who has tested positive can opt for a simple genetic test to find out whether they've got the faulty gene," says Professor Evans.

If you don't have a relative who has tested positive for a faulty gene, family history needs to be strong to justify testing.

"Looking at family history means looking at the combinations of breast and ovarian cancer, and young-onset [younger than 50] breast cancer in the family," says Professor Evans.

"For example, a family history of three women in their 30s or four women in their 40s having cancer would easily qualify someone for genetic testing."

What if a woman without cancer has a faulty gene?

If you have a faulty gene, you have a high lifetime risk of breast and ovarian cancer.